The present invention relates to pressure sensors and in particular, concerns a capacitive pressure sensor suitable for use in automotive seats for safety applications.
Automotive occupant safety systems are increasingly employing occupant sensors as well as crash sensors to control safety restraint deployment. For example, conventional air bag deployment strategies do not consider the weight, size or position of the vehicle occupant(s). Recently, automotive safety systems have included occupant sensors to aid in determining whether air bag deployment would cause more harm than benefit and/or whether staged air bag deployment is desirable.
Conventional seat sensors comprise one or more pads including force sensitive resistive films. Variable capacitive sensors are also known. In capacitive sensors, flexible plate-like capacitor poles are spaced apart from one another by a compressible layer of synthetic material. When the capacitor sandwich is acted upon by pressure, elastic compression of the synthetic layer results in the capacitor poles approaching one another. Thus, the capacity of the capacitor varies as a function of the pressure. The existence of a force and its magnitude can then be inferred from the change in capacitance. Similarly, when the force is removed, the compressed layer of synthetic material returns the capacitor poles to their initial position.
Such known sandwich-type capacitive sensors have several drawbacks. Primarily, the thickness of the synthetic layer increases the thickness of the sensor and reduces the flexibility of the sensor as well. Thus, there is a need for a capacitive pressure sensor having improved flexibility and reduced thickness as compared to conventional capacitive pressure sensors.
The present invention overcomes the drawbacks associated with known sandwich-type capacitive pressure sensors through the provision of a twopole capacitive sensor wherein one of the poles is bent over a flat portion of the other to create a bent edge. The two poles are spaced apart from each other to form a capacitor and the degree of opening between the two poles created in the region of the bent edge varies as a function of the force applied to the sensor. The degree of opening is the bending radius or bending angle of the pole which is bent.
An advantage of the present invention is that a compressible intermediate layer of synthetic material is unnecessary. The capacitor poles are spaced apart from one another by an electrically nonconductive fluid such as air. This makes the device more flexible, lighter and thinner than conventional sandwich-type capacitive sensors.
Another advantage is that the sensor is reset by the elastically deformed bent edge. No additional sensor resetting devices are required.
A further advantage is that the sensor is less complex and therefore less expensive to manufacture than prior sensors.
Other advantages and features of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.